Method for preparing surfaces of carbon fiber-reinforced plastics for further processing into supporting structural parts

ABSTRACT

The invention relates to a method for preparing surfaces of carbon fiber-reinforced plastics for further processing into supporting structural parts of aircraft by abrasive removal of the surface. In order to reduce the risk of cutting fibers during grinding while allowing greater strength of the bond so as to decrease the amount of fashioning to be done, the sand-blasting process is carried out by means of an abrasive that is distributed in dried air. A sharp-edged corundum grain which preferably has a grain size ranging between 0 and 90 m is used as an abrasive.

The invention relates to a method for preparing plastic surfaces,preferably surfaces of carbon fiber reinforced plastics, for furtherprocessing to produce supporting structural parts, particularlysupporting surfaces of aircraft, by means of abrasive removal of thesurface.

Such prepared surfaces are needed in modern aircraft construction inorder to be able to build even lighter structures, by means ofprocessing carbon fiber reinforced plastics. For example, the bondsurfaces of the upper and lower shell of supporting surfaces must bepretreated carefully, before they are bonded together to produce thefinished wing. In this connection, frames and stringers must also belaminated in between the shells.

The structural parts, which are subject to extreme stress in operation,are not allowed to fail. However, based on experience, bond points arefrequently the cause for intolerable weakening of the structural part,because of defect points that have less strength than the basicmaterial.

To avoid such defect points, the surfaces that are intended for furtherprocessing by means of bonding are first ground, and complicated qualityassurance measures are taken, in order to check the result of grinding.

When grinding laminated rough surfaces, there is the risk that thefibers that determine the strength are damaged by the grinding process.For this purpose, optical inspections are required, since fibers thatare ground through are impermissible. In addition, a measurement of thesurface resistance is made.

To determine contaminated points, a water-break test is performed, whichtakes advantage of the difference in surface tension of ground andcontaminated surfaces, in order to be able to make a statement about thequality of the result of grinding. In this connection, the bond surfaceis completely wetted using de-ionized water. Subsequently, the surfaceis set vertically, so that the water can flow off. In the case of awell-cleaned surface, the water flows off evenly. Water beads form atdirty points. Such points must be additionally ground. The parts mustfirst be dried in an oven, in order to remove the water, i.e. anyresidual moisture.

The space requirement for the grinding machines required for theseprocessing steps must be taken into consideration as early as in thedesign of the structural parts. In the interior space of the wings, inparticular, the ability to reach narrow dead-end-type spaces with thegrinding machine exists only with difficulty.

It is the task of the invention to propose a method for preparingplastic surfaces, preferably surfaces of carbon fiber reinforcedplastics, for further processing to produce supporting structural parts,particularly supporting surfaces of aircraft, by means of abrasiveremoval of the surface, which method reduced the risk of cutting of thefibers but, at the same time, makes greater strength of the bondpossible, with less finishing.

This task is accomplished, in the case of a method of this type, in thatthe material removal takes place by means of an abrasive sandblastingagent that is distributed in a gaseous fluid. If the time that isrequired in order to remove the surface, usually down to the fiber, i.e.to destroy it, is measured, about one-tenth of this time is surprisinglysufficient to achieve a treatment of the surface that is sufficient forthe bond. The quality of this surface actually surpasses that of aground surface. In fracture tests, the components fail not at the bondpoint, but rather in the basic material, and this significantlyincreases the ability of the bonded structural parts to withstandstress.

Contamination of the mechanically cleaned surface with the fluid isadvantageously avoided in that the gaseous fluid is dried air from whichoil has been removed.

Particularly durable bonds are achieved if a sharp-edged corundum grain,preferably in a grain size range between 0 and 90 μm, particularlybetween 17 and 62 μm, is used as the sandblasting agent.

An advantageously uniform working result can be achieved in that thesandblasting agent is applied to the surface to be treated by means of alow-pressure jet, preferably from a pressure boiler at less than 2.5bar.

The same purpose is served by the measure of applying the sandblastingagent to the surface to be treated by means of an eddy jet nozzle. Therotating jet of the eddy jet nozzle prevents the formation ofdisadvantageous so-called hot spots.

An optical inspection of the work result is advantageously sufficient,in order to assure sufficient quality of the surface. The treatment withthe sandblasting agent takes place as long as necessary in order toachieve an almost cloud-free, matt, treated surface. This quality isadvantageously defined analogous to DIN EN ISO 12944-4 SA 2.5, by way ofthe work result. Complicated water-break tests with drying steps inlarge, complicated ovens can therefore be eliminated.

For finishing the treated surface, it is surprisingly sufficient if thetreated surface is blown off with dried and oil-free air, in order toremove the sandblasting agent. Any remaining individual grains of thesandblasting agent that might have penetrated deeper into the surfacesurprisingly do not have a disadvantageous effect.

For quality assurance, it is sufficient that a quality inspection of thetreated surface takes place by means of measuring a surface resistanceand/or by means of an optical inspection using a comparison sample.

The reproducibility of the work result is additionally improved if thetreatment takes place in an air-conditioned, preferably draft-freeenvironment.

Before transport of the treated parts to further processing, it isprovided that the surface is vacuum-packed for transport. Such packagingis already sufficient to transfer the work process according to theinvention from the dust-sensitive component assembly to outside vendors.

The statistical stability of the work result, with its-qualitativeimprovements, is further improved if the sandblasting agent is notre-circulated. In the treatment of the sandblasting agent that isnormally usual in sandblasting technology, it cannot be precluded thatremoved particles of the basic material remain in the sandblastingagent. Treatment by means of magnetic separators is not possible in thecase of the plastics to be treated by means of the method according tothe invention, since they are not magnetic. They must therefore betreated by means of wind-sifting or flotation. Eliminating suchtreatment also eliminates these method steps.

The task is also accomplished by a semi-finished product, preferablymade of carbon fiber reinforced plastic, having a plastic surfaceprepared for further processing to produce supporting structural parts,particularly supporting surfaces of aircraft, wherein the surface isfashioned by means of a treatment according to one or more of the abovemethod claims, by means of a sandblasting agent distributed in a jet ofa gaseous fluid.

For example, prepregs are pretreated in a sandblasting method accordingto the invention, for bonding with stringers, for example, onto outerskins, deep bonding, ribs, reinforcements, etc. The method causes aslight removal of material, without any impairment of the dimensionalaccuracy of the component. It cleans the surface by means of mechanicalremoval of the surface, but this removal can be metered precisely, inadvantageous manner. There is approximately ten times greater securityagainst damage to fibers of the plastic. The sandblasting method iscarried out dry, whereby the effect on the component surface isessentially determined by the selection of the sandblasting agent. Thesurface roughness is primarily determined by the sandblasting agent andthe pressure. Particularly good results were achieved when using an eddyjet nozzle having the type designation Roto Soft Blast Mate Etc RotoSoft jet nozzles DK 8.5/M, in which the feed of sandblasting agent wasset in the scale 2 cm metering valve. The supply of compressed air wasprovided at 2.5 bar, using a pressure jet boiler from the Blast Matecompany. The sandblasting agent used was high-quality corundum, white,0.07 to 0 mm. After an almost cloud-free blasting image was achieved, inanalogy to DIN EN ISO 12944-4 SA 2.5, the only thing that took place wasdust removal from the surface by means of purified compressed air.

In the final analysis, the quality of the surface produced in thismanner far surpassed the quality previously achieved by means ofgrinding.

1. Method for preparing plastic surfaces, preferably surfaces of carbonfiber reinforced plastics, for further processing to produce supportingstructural parts, particularly supporting surfaces of aircraft, by meansof abrasive removal of the surface, wherein the material removal takesplace by means of an abrasive sandblasting agent that is distributed ina gaseous fluid.
 2. Method for preparing plastic surfaces for furtherprocessing to produce supporting structural parts, as recited in claim1, wherein the gaseous fluid is dried air from which oil has beenremoved.
 3. Method for preparing plastic surfaces for further processingto produce supporting structural parts, as recited in claim 1, wherein asharp-edged corundum grain, preferably in a grain size range between 0and 90 μm, particularly between 17 and 62 μm, is used as thesandblasting agent.
 4. Method for preparing plastic surfaces for furtherprocessing to produce supporting structural parts, as recited in claim1, wherein the sandblasting agent is applied to the surface to betreated by means of a low-pressure jet, preferably from a pressureboiler at less than 2.5 bar.
 5. Method for preparing plastic surfacesfor further processing to produce supporting structural parts, asrecited in claim 1, wherein the sandblasting agent is applied to thesurface to be treated by means of an eddy jet nozzle.
 6. Method forpreparing plastic surfaces for further processing to produce supportingstructural parts, as recited in claim 1, wherein a treatment with thesandblasting agent takes place as long as necessary in order to achievean almost cloud-free, matt, treated surface.
 7. Method for preparingplastic surfaces for further processing to produce supporting structuralparts, as recited in claim 1, wherein the treated surface is blown offwith dried and oil-free air, in order to remove the sandblasting agent.8. Method for preparing plastic surfaces for further processing toproduce supporting structural parts, as recited in claim 1, wherein aquality inspection of the treated surface takes place by means ofmeasuring a surface resistance and/or by means of an optical inspectionusing a comparison sample.
 9. Method for preparing plastic surfaces forfurther processing to produce supporting structural parts, as recited inclaim 1, wherein the treatment takes place in an air-conditioned,preferably draft-free environment.
 10. Method for preparing plasticsurfaces for further processing to produce supporting structural parts,as recited in claim 1, wherein the surface is vacuum-packed fortransport.
 11. Method for preparing plastic surfaces for furtherprocessing to produce supporting structural parts, as recited in claim1, wherein the sandblasting agent is not re-circulated. 12.Semi-finished product, preferably made of carbon fiber reinforcedplastic, having a plastic surface prepared for further processing toproduce supporting structural parts, particularly supporting surfaces ofaircraft, wherein the surface is fashioned by means of a treatmentaccording to claim 1, by means of a sandblasting agent distributed in ajet of a gaseous fluid.